1. Technical Field
The present disclosure relates to control systems for power electronics devices.
2. Description of the Related Art
Generally, power supply/converter applications require the power supply to operate within a well-defined operating area bounded by various electrical limits (e.g., voltage, current, power, resistance and conductance). The operation of power supplies may be controlled to operate within an operating area to either protect the power supply, protect a load coupled to the power supply, or for some desired control effect. Depending upon the application, the power supply may, for example, be required to provide a constant power output that does not exceed a specified current limit. Further, some power supply applications require the power supply to be capable of effectively switching between operating modes. For example, some power supplies may switch between providing a constant voltage to providing a constant current.
FIG. 1 shows a simplified schematic diagram of a DC/DC power converter 10. The power converter 10 operates by repeatedly opening and closing a power switch 12. The switch 12 may, for example, be a metal oxide semiconductor field-effect transistor (MOSFET). In some implementations, closing the switch 12 causes a current to flow from a direct current (DC) input source 14 through a winding of a magnetizable part 16 (e.g., a primary winding of a transformer, an inductor). In one example, a rough DC voltage is present between terminals of the switch 12. In another example, an alternating current (AC) line voltage may, for example, be rectified by a bridge rectifier (not shown) and an associated smoothing capacitor (not shown) to provide rectified and smoothed rough DC voltage to the terminals of the switch.
When the switch 12 is closed, the current that flows through the magnetizable part 16 may cause energy to be stored in the magnetizable part. The switch 12 is then opened. When the switch 12 is opened, energy stored in the magnetizable part 16 is transferred to an output node 18 of the power converter through a rectifier and output filter 20 (e.g., a diode and a capacitor). The current may charge the output filter 20. In steady state operation in a constant voltage (CV) mode, the switch 12 may be switched to open and close rapidly and in such a manner that the output voltage VOUT on the output filter 20 at the output node 18 remains substantially constant.
The power converter 10 includes a controller 22 that controls the opening and closing of the switch 12. For example, the controller 22 may take the form of a pulse width modulation (PWM) controller.
The power converter 10 operates in two control modes that utilize servo feedback control: constant voltage (CV) mode and constant current (CC) mode. In the CV operational mode, the output voltage VOUT is sensed by a voltage sense circuit 24 (e.g., a resistor divider). An output node 26 of the voltage sense circuit 24 is coupled to an inverting input terminal of a voltage control amplifier 28.
The voltage control amplifier 28 may be coupled to a positive power supply and a negative power supply (e.g., VCC and ground, +VCC and −VCC, etc.). The voltage control amplifier 28 compares the voltage at the output node 26 of the voltage sense circuit to a reference voltage VREF1 coupled to a non-inverting input terminal and outputs the result of the comparison onto an output terminal of the voltage control amplifier 28 at node 29 (VEA).
A feedback compensation circuit 30 is coupled between the inverting input terminal of the voltage control amplifier 28 and the output terminal. The feedback compensation circuit 30 includes a compensation or feedback capacitor 32 and a feedback resistor 34 connected together in series. If the comparison of the voltage control amplifier 28 inverting input is lower than its non-inverting input (i.e., VOUT is below regulation), then the output voltage of the amplifier 28 will increase to the required voltage at the common control node 38 to maintain regulation of VOUT. If the comparison of the amplifier 28 inverting input is greater than its non-inverting input (i.e., VOUT is above regulation), then the output voltage of the amplifier 28 will decrease to the required voltage at the common control node 38 to maintain regulation of VOUT. In both cases, this error voltage (VCONTROL) at the common control node 38 is indicative of the output voltage VOUT at the output node 18. The pull-up resistor 36 provides the sourcing current for the amplifier 28. An input of the controller 22 is coupled to the common control node 38 and, based on the received error voltage, the controller controls the on/off duty cycle of the switch 12 to regulate output voltage VOUT.
In the CC operational mode, the current IOUT being supplied by the power converter 10 is sensed by a current sense circuit 42 (e.g., a sense resistor). The voltage on an output node 44 of the current sense circuit 42 is sensed by a current control amplifier 46. The current control amplifier 46 may be coupled to a positive power supply and a negative power supply (e.g., VCC and ground, +VCC and −VCC, etc.).
A feedback compensation circuit 48 is coupled between the inverting input terminal of the voltage control amplifier 46 and the output terminal at node 47 (VCA). The feedback compensation circuit 48 includes a compensation or feedback capacitor 50 and a feedback resistor 52 connected together in series.
If the voltage output by the current sense circuit 42 is greater than a predetermined value, then the current control amplifier 46 causes the voltage on an output terminal thereof to decrease to a relatively low voltage as necessary at the common control node 38 (VCONTROL) to maintain the predetermined limit of the output load IOUT. The pull-up resistor 36 provides the sourcing current for the amplifier 46. The voltage sensed by the controller 22 is therefore indicative of the magnitude of the output current IOUT. Based on the common control node 38 voltage, the controller controls the on/off duty cycle of switch 12 to regulate output current IOUT.
In this example, the blocking diodes 40 and 54 are configured such that a minimum error signal is selected and provided to the controller 22. Thus, the diodes may be referred to as being in an “ORed” or “ORing” configuration.
The power converter 10 operates either in the CV mode or in the CC mode, depending on the loading condition. In one example, if the output current exceeds a specified current (e.g., a short circuit or other overcurrent condition), then the power converter 10 operates in the constant current mode. Otherwise, the power converter 10 operates in the constant voltage mode. Thus, in this example, the CC mode acts as a current limiter to protect one or more components of the power converter, such as the switch.
In the power converter 10 of FIG. 1, during transitions between modes of operation (e.g., transition from CV to CC mode), undesirable transients may occur. In particular, during a short circuit condition at the output node 18, high transient input current may occur that can damage the switch 12 or other circuitry. Further, excessive input current ripple and output voltage ripple can occur during startup of the power converter 10, especially when delivering power to a capacitive load.